The present invention relates generally to gas discharge devices, and more particularly to the cathode construction found in such devices.
A laser typically employs an unheated or cold cathode which is secured to a laser body or housing as a source of electron emission for laser operation. The body of the laser may be composed of glass or glass like materials, generally having low coefficients of thermal expansion. The cathode may be composed of a metal or metal-alloy material, for example aluminum, well known in the art. The cathode is generally secured to the laser body by a gas tight seal, and is adapted to be connected to a negative electric potential source.
In gas lasers having a limited gas supply, cathode sputtering is one of the major causes of shortened laser life. In a helium-neon gas laser, positively charged gas ions of the plasma are attracted to the negatively charged cathode, and release negatively charged electrons. Unfortunately, the positively charged ions can dislodge cathode material molecules from the active electron emitting surface of the cathode. This phenomenon is usually referred to as cathode sputtering. For gas laser applications, cathode sputtering results in decreased laser life. As a result of cathode sputtering, the dislodged cathode material can, in turn, trap or bury lasing gas molecules into the active emitting surface walls of the cathode. If the supply of gas is limited, the gas molecule burying action, caused by sputtering, can eventually reduce the available gas ions to the point that lasing action ceases.
Metallic cathodes, particularly aluminum cathodes, have been widely used in the art for gas lasers. An aluminum cathode generally has the cathode emitting surface coated with a thin layer of oxide to prevent cathode sputtering. During the cathode manufacturing process, a layer of oxide is formed naturally by exposing a cleaned aluminum cathode emitting surface to an oxygen plasma with the aluminum cathode connected as the cathode in an electrical circuit. A thin layer of oxide is formed on the aluminum electron emitting surface due to the pressure of oxygen and oxygen ions hitting the cathode surface.
Aluminum cathodes having the oxide layer have improved laser life above that of uncoated aluminum due to increasing the resistance to sputtering. This is so since the oxide layer is generally harder than the aluminum. Nevertheless, irregularities in the emitting surface of the cathode can result in localized ion flow which in time breaks down the oxide layer, and begins localized sputtering of the cathode resulting in extinction of the laser.
Further, in some laser applications, it is desirable that the cathode have a very low thermal coefficient of expansion so that it can be secured to a laser body or block which has a very low coefficient of thermal expansion. A body of a laser comprised of quartz like products such as Zerdur and Cervit has a very low coefficient of thermal expansion. In these circumstances, it is highly desirable that the coefficient of thermal expansion of the cathode be as low as possible and preferably match the coefficient of thermal expansion of the laser body.